In the last ten years, numerous studies have demonstrated the major role played by melatonin (N-acetyl-5-methoxytryptamine) in a large number of physiopathological phenomena and in the control of the circadian rhythm, but melatonin has a rather short half-life owing to the fact that it is rapidly metabolised. Great interest therefore lies in the possibility of making available to the clinician melatonin analogues that are metabolically more stable and have an agonist or antagonist character and of which the therapeutic effect may be expected to be superior to that of the hormone itself.
In addition to their beneficial action in respect of circadian rhythm disorders (J. Neurosurg. 1985, 63, pp. 321–341) and sleep disorders (Psychopharmacology, 1990, 100, pp. 222–226), ligands of the melatoninergic system have valuable pharmacological properties in respect of the central nervous system, especially anxiolytic and antipsychotic properties (Neuropharmacology of Pineal Secretions, 1990, 8 (3–4), pp. 264–272), and analgesic properties (Pharmacopsychiat., 1987, 20, pp. 222–223), and also for the treatment of Parkinson's disease (J. Neurosurg. 1985, 63, pp. 321–341) and Alzheimer's disease (Brain Research, 1990, 528, pp. 170–174). The compounds have also demonstrated activity in relation to certain cancers (Melatonin—Clinical Perspectives, Oxford University Press, 1988, pp. 164–165), ovulation (Science 1987, 227, pp. 714–720), diabetes (Clinical Endocrinology, 1986, 24, pp. 359–364), and in the treatment of obesity (International Journal of Eating Disorders, 1996, 20 (4), pp. 443–446).
Those various effects are exerted via the intermediary of specific melatonin receptors. Molecular biology studies have demonstrated the existence of a number of receptor sub-types that are capable of binding that hormone (Trends Pharmacol. Sci., 1995, 16, p. 50; WO 97.04094). It has been possible for some of those receptors to be located and characterised for different species, including mammals. In order to be able to understand the physiological functions of those receptors better, it is of great advantage to have available selective ligands. Moreover such compounds, by interacting selectively with one or another of those receptors, may be excellent medicaments for the clinician in the treatment of pathologies associated with the melatoninergic system, some of which have been mentioned above.
In addition to being new, the compounds of the present invention exhibit a very strong affinity for melatonin receptors and/or a selectivity for one or another of the melatoninergic receptor sub-types.